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    • Cy3-UTP: Photostable RNA Labeling Reagent for High-Resolu...

    2026-02-16

    Cy3-UTP: Photostable RNA Labeling Reagent for High-Resolution Molecular Probing

    Executive Summary: Cy3-UTP is a Cy3-labeled uridine triphosphate analog optimized for RNA labeling and detection. Its incorporation during in vitro transcription enables generation of highly photostable, bright fluorescent RNA (APExBIO, product page). Cy3-UTP facilitates real-time RNA imaging, RNA-protein interaction assays, and site-specific RNA tracking at single-nucleotide resolution (Wu et al., 2021). The reagent is supplied as a water-soluble triethylammonium salt, with a recommended storage condition of -70°C to preserve stability. This article reviews the mechanistic rationale, empirical evidence, and application boundaries for Cy3-UTP as a molecular probe for RNA.

    Biological Rationale

    RNA molecules play central roles in gene regulation, catalysis, and molecular recognition. Accurate mapping of RNA structure, localization, and interactions requires site-specific labeling with stable, bright fluorophores. Cy3-UTP is a modified nucleotide that enables covalent incorporation of the Cy3 dye into RNA during in vitro transcription. The Cy3 fluorophore is valued for its high quantum yield, excitation at ~550 nm, and emission at ~570 nm, making it compatible with standard fluorescence microscopy and detection platforms (internal review). Unlike post-synthetic labeling, enzymatic incorporation of Cy3-UTP ensures uniform, reproducible labeling and preserves RNA integrity.

    Mechanism of Action of Cy3-UTP

    Cy3-UTP is structurally analogous to uridine triphosphate, with the Cy3 dye covalently attached to the uracil base. During in vitro transcription, RNA polymerases (e.g., T7, SP6) incorporate Cy3-UTP in place of natural UTP at designated positions. This process results in fluorescently labeled RNA strands suitable for downstream applications (see in-depth analysis). The photostability of Cy3 minimizes photobleaching, enabling prolonged imaging sessions. Incorporation efficiency depends on enzyme, template sequence, and reaction conditions (e.g., 37°C, pH 7.5–8.0, 1–10 mM MgCl₂). The B8330 reagent is supplied as a triethylammonium salt, readily soluble in water, with a molecular weight of 1151.98 (free acid form).

    Evidence & Benchmarks

    • Site-specific labeling of adenine riboswitch RNA at single-nucleotide resolution using Cy3-UTP and PLOR (Position-Selective Labeling of RNA) enables real-time tracking of ligand-induced conformational changes (Wu et al., 2021, iScience).
    • Cy3 dye exhibits excitation at 550 nm and emission at 570 nm under standard buffer conditions (pH 7.4, 25°C), with high brightness and quantum yield (>0.15) (APExBIO datasheet).
    • Fluorescent RNA generated using Cy3-UTP enables sensitive detection in stopped-flow fluorescence assays, requiring only nanomoles of labeled sample (Wu et al., 2021).
    • Cy3-UTP outperforms conventional non-fluorescent UTP in RNA localization, FRET, and RNA-protein interaction assays due to superior photostability and reproducibility (internal protocol review).
    • RNA labeled with Cy3-UTP retains functional secondary structure and ligand responsiveness, as validated in riboswitch folding studies (Wu et al., 2021).

    Applications, Limits & Misconceptions

    Cy3-UTP serves as a core tool in several advanced RNA research workflows:

    • Fluorescent imaging of RNA: Enables single-molecule and bulk imaging in vitro and in cells, supporting studies of RNA localization and trafficking (see strategic integration).
    • RNA-protein interaction studies: Facilitates real-time observation of RNA conformational changes and binding kinetics using FRET, stopped-flow, or single-molecule assays.
    • RNA detection assays: Increases sensitivity and specificity in hybridization-based detection, including microarrays and Northern blots.
    • Tracking RNA dynamics: Supports high-resolution tracking of riboswitches and other functional RNAs in response to ligands (translational insight article).

    Common Pitfalls or Misconceptions

    • Not suitable for in vivo transcription: Cy3-UTP is generally not incorporated efficiently by endogenous polymerases in live cells; it is optimized for in vitro systems.
    • Long-term storage of solution is discouraged: The reagent should be freshly prepared and protected from light; aqueous solutions degrade over days even at -20°C.
    • May affect RNA folding if incorporated at critical structural motifs: Excessive labeling or labeling within key functional domains may perturb RNA structure.
    • Compatibility with enzymes: Some polymerases or templates may exhibit reduced efficiency or bias with bulky Cy3 modifications.
    • Not a quantitative substitute for mass spectrometry or radioactive labeling: Fluorescence labeling provides qualitative and semi-quantitative but not absolute quantification.

    This article extends previous internal reviews (photostability analysis) by providing direct, citation-backed benchmarks and clarifying the mechanistic limits of Cy3-UTP in RNA biology workflows.

    Workflow Integration & Parameters

    Incorporation of Cy3-UTP is compatible with standard in vitro transcription kits (e.g., T7, SP6). Recommended reaction conditions: 1× transcription buffer, 1–2 mM Cy3-UTP, 1–4 mM NTPs, 37°C, pH 7.5–8.0, 1–10 mM MgCl₂, 30–120 minutes. For position-selective labeling (PLOR), Cy3-UTP is introduced at defined cycles to achieve single-nucleotide resolution (Wu et al., 2021). Post-transcription, labeled RNA is purified by standard methods (e.g., ethanol precipitation or spin columns). Storage: -70°C, protected from light. Use immediately after dilution to avoid hydrolysis or photodegradation. For advanced applications such as RNA folding kinetics or riboswitch dynamics, Cy3-UTP-labeled RNA is directly compatible with stopped-flow and single-molecule FRET platforms.

    For detailed workflow enhancements and troubleshooting, see "Cy3-UTP (SKU B8330): Enhancing RNA Labeling Reliability" (which this article updates by incorporating recent peer-reviewed evidence and mechanistic clarifications).

    Conclusion & Outlook

    Cy3-UTP (APExBIO, B8330) represents a robust, photostable, and reproducible solution for fluorescent RNA labeling in vitro. Its utility in real-time RNA tracking, conformational analysis, and RNA-protein interaction studies is supported by both product validation and peer-reviewed research (Wu et al., 2021). While not suitable for direct in vivo use, Cy3-UTP remains a standard for advanced RNA biology research requiring sensitivity, specificity, and workflow reliability. Continued methodological advances (e.g., improved PLOR protocols) will further expand its applications in single-molecule and systems-level RNA analysis.